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I see that your "all conditions" is indeed the recommendation everywhere. Thanks. Now, should one take the internal tire measurer provided by the Bolt as being the final arbiter of psi? My plug-in inflator shows 40 but then the Bolt screen shows 34 or 35. It would seem reasonable that the internal measurer is accurate, so I guess I should pump air in until the Bolt screen says 38.
 

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stanwagon-

I am having the same tire pressure issue. I am at 8800ft. I have several tire pressure gauges, they all show 39psi. The app shows 34. Maximum tire pressure is 44 on the stock tires. I am thinking I will be at or above the max on my tire gauge, to get the app at 38. For now I am going with my tire pressure gauge. I do not know the correct answer.
 

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stanwagon-

I am having the same tire pressure issue. I am at 8800ft. I have several tire pressure gauges, they all show 39psi. The app shows 34. Maximum tire pressure is 44 on the stock tires. I am thinking I will be at or above the max on my tire gauge, to get the app at 38. For now I am going with my tire pressure gauge. I do not know the correct answer.
This is just a guess and I haven't run the numbers to see if it makes sense, but I would go with the external gauge. What you really want to know is the difference between atmospheric and internal pressure and your external gauge will show you that. If the TPMs where mounted at see level, they are probably calibrated against that atmospheric pressure and thus read a lower differential pressure. I'd be curious to find out how the TPMs are really calibrated. 8800 ft is pretty extreme, probably way outside the expected usage range.
Hope this makes sense.

Ron
 

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Hah: I was just going to post this: In another car I bot recently I was taking LOW PRESSURE warnings at trailheads above 11000 feet. I called dealer. He said: "Oh, you need to recalibrate the tire sensor." and there was a key sequence to do that.

So I will check the manual, etc., to find out if a reset exists for Chevy Bolt. The air pressure here is 70% that of sea level.
 

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Thank you. Apologies for going off-topic back there. But this does seem to be an interesting topic. I am actually at a total loss as to how to know the pressure in a tire.

1. My plug in inflator: I pumped one up until the device dial said 42.
2. After #1 my Bolt 4-wheel display says 34 34 34 34
3. A little pen-like measurer says 36.

The dealer said there is no way to recalibrate the sensor. He did not really say that there is a problem at altitude, but that was the case for my Toyota Highlander recently (gave LOW PRESSURE warnings until I did a sensor reset).

I did look at other forums where the issue of altitude (I am at 10000) is discussed.

I am going to assume #3 above it the most accurate. But the huge discrepancies above are annoying to a numbers guy.
 

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But the huge discrepancies above are annoying to a numbers guy.
Sea level pressure is equal to 14.7 psi. 10,000 feet above sea level the atmospheric pressure is 10.1 psi. Thus there's a 4.6 psi differential between the two. The internal tire sensor can't reliably measure gauge pressure being inside the tire. Your external pressure gauge always measures the difference between the internal tire pressure and the atmospheric pressure. That's why they call it gauge pressure.

The pressure reported from the tire sensor, ignoring the volume change of the tire, is actually absolute pressure and mathematically adjusted to sea level. So it measures 52.7 and someone has corrected it by 14.7 to display 38. The problem is, as you go up in altitude, the tire still has 52.7 psia (meaning absolute pressure) inside and now is over inflated. So when you remove air you drop the absolute pressure of the tire to 52.7 - 4.6 to 48.1 psia. But with your gauge, you measure it as 48.1 - 10.1 = 38 psig. But the Bolt software is still mathematically adjusted to sea level and reports back that the pressure is 48.1 - 14.7 = 33.4 psi.

So all of this can be fixed with a software update. I don't know if the Bolt has a barometric pressure measurement. If it did, it could automatically adjust the tire pressure display on its own. Or it could use GPS and its location to discern altitude based on its geographical location.

Recalibration of the sensors is not necessary. Just needs the right correction factor based on altitude. All software.
 

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I am actually at a total loss as to how to know the pressure in a tire. .
Yes. I am using my favorite bicycle sliding plunger gauge to set the Bolt's tire pressure. I have an air compressor with an analog gauge on the tank, and another at the air chuck. They don't match. I have four bicycle pumps. They don't match. And the TPMS don't match each other when cold, but DO match when warmed up after driving awhile. I don't find this at all surprising. I worked as a tech for some years, and finding two lab grade thermometers that gave the same reading, out of six, was a miracle. Calibrating instruments, and whole labs to give matching results is a nightmare. I often laugh about people quibbling over advertised Cd for cars, run at different times, in different wind tunnels. Unless they hauled a standard model from lab to lab, and calibrated to that standard, before every run, the differences they argue about probably noise.
 

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Thanks XJ. We know the Bolt has internal GPS for its HOME setting on charging. That gives elevation very accurately these days, so a barometric measurer wouldn't be necessary. But you are saying quite simply that I just need to add 4.6 to the 34 I see on the screen. That is consistent with the idea that

A. the device in the tire is accurate for what it does (good)
B. the tires were properly inflated in the first place (good)

So that puts the matter to rest as far as users are concerned, tho GM clearly should fix the altitude correction.
 

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Good stuff Loose Bolt and XJ.

While it would be nice to be able to reset the reference pressure so that the reading displayed more accurately, it's pretty easy to just adjust in your mind. The main purpose of the gauge is to warn if pressure is critically low, and also to be a reference for consistency. If the gauge is under-reporting pressure by 6 PSI, that isn't a big deal as long as it's fairly consistent, and pressure isn't dropping over time.

I disagree with the flat 38 PSI in all conditions statement. Tire inflation is one of personal preference. Even manufacturers don't necessarily choose the inflation pressure that is highest performing; they take into consideration other factors such as road noise and ride comfort to strike a compromise of safety, comfort, performance, and fuel economy. If my goal were efficiency and range extension, I'd probably inflate to somewhere around 45 PSI, as I do in my Acura. On rough, bumpy roads I'd probably reduce the pressure to increase comfort, although if pinch flats were a concern then I'd increase the pressure. In general, I overinflate by about 10 PSI over factory to increase performance and economy at the cost of reduced ride comfort and increased noise.
 

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Discussion Starter #12
I overinflate by about 10 PSI over factory to increase performance and economy at the cost of reduced ride comfort and increased noise.
Tastes differ, but I certainly would never run a tire over the manufacturer recommended max.
Note that once your tire warms up, you would be at about 6 lbs. over the 45lb. max.
Oh, and BTW, 10 lb. overinflation will also shake the devil out of the car from the harder impacts, and will make the interior noisier sooner.
Good luck...
 

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I am actually at a total loss as to how to know the pressure in a tire.

1. My plug in inflator: I pumped one up until the device dial said 42.
2. After #1 my Bolt 4-wheel display says 34 34 34 34
3. A little pen-like measurer says 36.
I never trust the gauges on inflation devices, including the electric pump that came with the Bolt and the one on my good ol'-fashioned manual tire pump. They always seem to read high.

The pen-like gauges with the scale that slides out when you push it onto the valve stem are the most accurate, IMHO. Chances are that the discrepancy between that gauge and what the Bolt's display is showing you is partly due to rounding errors. My Canadian Bolt shows tire pressure in kPa, and the values shown are always multiples of 4 (i.e., it'll show 284 or 288 but it will never show 285, 286, or 287). So there's obviously a limit to the resolution of the TPMS sensor or the protocol that passes the information to the car.
 

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Tastes differ, but I certainly would never run a tire over the manufacturer recommended max.
Note that once your tire warms up, you would be at about 6 lbs. over the 45lb. max.
Oh, and BTW, 10 lb. overinflation will also shake the devil out of the car from the harder impacts, and will make the interior noisier sooner.
Good luck...
Exactly, which is why I preface my comments saying the matter is one of preference. I weigh the benefits of lower fuel consumption and better performance with the drawbacks of harsher ride, increased noise, and increased suspension loads when determining how much to overinflate.

As an aside, max pressure ratings can safely been ignored. The burst pressure of tires is several hundred PSI; much higher than most compressors are capable of producing. Of much greater concern is underinflation. My recommendation is to fill to as high a pressure as is comfortable, with the minimum being the vehicle manufacturers recommended pressure.
 

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OK. I think the fog is beginning to lift. The gauge inside the tire is in a sealed compartment. Let's ignore that one. When the car is at sea level 38 psi is recommended. When the car is brought to 10000 ft, that amount of air in the tire will cause a different shape to the tire. It will cause (am I right?) the sliding scale reader to read something like 42. So to get back to 38 against ambient air -- which surely is what is wanted -- one would have to let air out of the tires.

So the rule "38 in all conditions period" does mean "38 against ambient air -- and simply ignore the internal sealed-compartment gauge". Of course, I can learn that 34 on that gauge really means 38 (roughly) and, most important, I can watch it for changes.

But there is clearly room for confusion. It appears that on moving up 10K feet the pressure against ambient goes up and the pressure in the sealed-compartment goes down (b/c of software and subtraction of sea level ambience). Whew.
 

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(b/c of software and subtraction of sea level ambience).
Actually, the software doesn't do anything but read back the pressure. The original calibration was done with sea level as its reference (they used a gauge). The sensor is only capable of reading the absolute pressure (because it is closed inside a tire). But the software shows the pressure as gauge which assumes the sea level correction (or possibly some other reference as a compromise). The software needs to do more than just have a fixed calibration curve that is only adjusted for sea level. If they had calibrated the device to report back in absolute pressure, then gauge pressure can always be calculated and displayed correctly by knowing the local atmospheric pressure (ambient). Instead, it shows the pressure that only reads accurate at sea level.

Kind of ironic that most of our nation's standards are kept in Boulder Colorado. Maybe they should have used Colorado's ambient pressure as the reference then @stanwagon wouldn't be having issues with the tire pressure.

https://www.nist.gov/pao/nist-boulders-precision-measurement-laboratory
 

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Precisely right Stanley.

Just to complicate things, another factor not yet mentioned is temperature. Pressure increases with increasing temperature, or decreases with decreasing temperature. All else held constant, people will generally observe lower tire pressures in the winter than in the summer due to lower temperature. It may be worthwhile to add air to the tires in the winter, and let some out in the spring or summer.

Personally, I don't bother much with temporary changes in elevation (like visiting someone at elevation for a week), or changes in season. I'll check my tire pressure about twice per year and adjust accordingly. If changes in handling present themselves, it will also prompt me to investigate and remedy the issue before it becomes a problem.
 

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It's Stanley. So my name is identical to that old car whose pic I show: A "Stanley Wagon".
Years ago I learned that barometric altimeters are calibrated to the yearly average b/c of temp. fluctuations. This has huge implications for pilots in pre-GPS days. A pilot might think he was at 15000 feet to get over the Colo mts but he could have been actually at 14400, potential disaster.

A barometric altimeter will be (at least my instrument) 4% optimistic in winter and 8% pessimistic in summer in terms of feet gained (assuming calibration and constant pressure).

Of course, I knew about temperature and tire pressure, but clearly I had a bit to learn about the big picture. Now: If I ask a physics colleague: what happens to the measured tire pressure on going from 0 to 10000 ft: it is a trick question, since the method of measuring is not specified in the question.

The only really annoying problem is if the LOW warning comes on when it shouldn't. That has happened quite a bit in cars up here in the years since the internal measurers, and it is annoying since one then inflates when it might not have been necessary. I do like that the numbers on the Bolt are tied to specific tires. That was not the case on my previous 2 or 3 cars.
 

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Now: If I ask a physics colleague: what happens to the measured tire pressure on going from 0 to 10000 ft: it is a trick question, since the method of measuring is not specified in the question.
Actually, the pressure does drop slightly because the tire will grow without the added external pressure. The ideal gas equation is PV=nRT. The number of molecules of air is the same. Assuming the same temperature. The pressure will drop because the volume of the tire will increase. So your physics friend might not be fooled. Although the drop in absolute pressure is not very much.
 

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It's Stanley. So my name is identical to that old car whose pic I show: A "Stanley Wagon".
Years ago I learned that barometric altimeters are calibrated to the yearly average b/c of temp. fluctuations
Apologies Stanley. I was inspired by the photo to Google the Stanley Steemer and realized I had misspelled your name, but forgot to edit my post.

Anyhow, it was clear to me that you had understood the pressure conversation, and my post was more directed to the larger audience who might not have considered the influence temperature has on pressure. I'm not good at making it clear to whom my audience is, among many other breaches of social etiquette.

Anyone wish to discuss the gimmick of nitrogen filled tires? There's a mostly pointless added expense some tire companies advertise.
 
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